The present invention relates generally to threaded fasteners such as screws, bolts and nuts and, more particularly, to anti-cross threading fasteners.
Threaded fasteners are used in the construction or fabrication of most articles of manufacture such as machines, automobiles, trains, planes, engines, etc., and such threaded fasteners may take the form of bolts, screws, studs, rods (all of these terms being used interchangeably herein) or other substantially round members having uniform, nonuniform or tapered external helical threads that are threadedly engaged into mating threaded fasteners such as nuts, bolts or holes having substantially matching internal helical threads. To ensure proper engagement between externally and internally threaded fasteners, it is important that the longitudinal axes of these parts are in substantially collinear alignment before threaded engagement occurs in order to prevent the occurrence of cross threading.
Cross threading generally occurs when there is a misalignment between the externally threaded member, typically a screw or bolt, and the internally threaded member, typically a nut or other threaded hole. Specifically, cross threading is the result of the threads of the two members attempting to engage each other when out of alignment. When this occurs, the two members are not collinear with each other and wedging of the threads will occur as the threaded helixes are rotated against each other. If the rotation continues, as is often the case, the threads on one or both members will become structurally damaged.
While the problem of cross threading is significant under any circumstances in terms of engendering costly downtime and disruption in production, the problem of cross threading is compounded in situations where either the threaded stud or nut is first attached, for example by welding, to a large panel-like or other structural member which then facilitates threaded attachment of another part thereto. In other words, it is typical for weld nuts to be welded to such panels or structural members in one production step (e.g. during the manufacture of an automotive vehicle) to provide an internally threaded means for securing another member to the first mentioned member in a subsequent manufacturing step. In this subsequent step, the cross threading problem is particularly significant since it requires the first mentioned structural member to be repaired and recycled for the purpose of reuse at a later time.
The present invention is directed to an anti-cross threading nut comprising a nut body having first and second open ends at opposite sides thereof and a generally cylindrical passage extending between the first and second open ends. A plurality of threads extend over a predetermined axial extent of the passage to define a threaded region. A remaining portion of the passage is unthreaded up to the first end to define an unthreaded counterbore. The first end defines an inlet opening into the passage for receiving a correspondingly threaded shank member. The inlet opening is radiused or chamfered to prevent cross threading engagement of the shank member with the inlet opening. Advantageously, the shank member proceeds through the unthreaded region which forces the shank to become substantially collinearly aligned with the threaded region of the nut before actual threaded contact occurs. In this manner, cross threading is effectively minimized or eliminated.
In the preferred embodiment, the first inlet opening is radiused. Optionally, though preferably, a second lead-in radius is disposed at the intersection between the threaded region and the unthreaded counterbore. This second lead-in radius further minimizes the possibility of cross threading by providing a smooth transition for the threads of the shank member to pass through the unthreaded counterbore into proper threaded engagement with the threaded region.
The length and diameter of the unthreaded counterbore is structured to limit misalignment between the threaded region and the correspondingly threaded shank member to less than about 10xc2x0, and preferably less than about 5xc2x0. As a result of extensive testing, it has been determined that there is a significant reduction in cross threading at less than such angles of misalignment.
According to one aspect of the present invention, the design of the counterbore is controlled by the thread pitch. In a preferred embodiment, the axial extent or depth of the unthreaded counterbore is equal to at least about four times the thread pitch. This tends to force the before mentioned stud member and threaded region of the nut into correct collinear alignment wherein the threads of the stud and nut members will rotationally engage each other to properly fasten as intended.
The first lead-in radius is preferably at least about 1.5 times the pitch of the nut threads. This enables the thread surface of the stud to smoothly pass into the entrance opening of the unthreaded counterbore without binding or cross threading contact therebetween.
The diameter of the counterbore is machined to equal the nominal diameter of the threaded region plus about 10% of the thread pitch in order to ensure sufficiently snug guiding contact which will enable the aforesaid substantially collinear alignment to occur at the time of threaded engagement.
The above mentioned features may be used in any type of nut applications, such as hex nuts, hex flange nuts, or even one or both ends of a rod coupler. In one preferred embodiment of the present invention, however, the above mentioned anti-cross threading features are formed in a weld nut that includes weld protrusions on a side thereof including the first open end. This enables the nut to be welded to a structural member to facilitate subsequent threaded connection of this member to another member by means of a connecting threaded stud. In this type of environment in which the nut is previously securely welded to a larger structural member, the anti-cross threading features of the invention are particularly important since cross threading has more damaging and costly consequences.
In weld nut applications, the first open end is preferably formed on an elongated pilot section containing the unthreaded counterbore so as to axially offset the open end from the weld protrusions in a direction away from the threaded region of the nut. This pilot section advantageously prevents weld spatter from entering the unthreaded counter bore and possibly contaminating the threads.
In this weld nut embodiment, the weld nut is preferably formed with a material that has been hardened during the nut manufacturing process such as by application of heat in a known manner. By hardening the nut material, the nut threads also become hardened to provide added strength. In accordance with another feature of the invention, however, the hardened weld protrusions are selectively annealed to enable the weld protrusions to function properly to obtain appropriate welded contact between the nut and a supporting structural member. It will be appreciated that the feature of selectively annealing the weld protrusions in an otherwise hardened nut may be used with or without the above mentioned anti-cross threading features.
Accordingly, in accordance with a different aspect of the invention, there is provided a weld nut comprising a nut body formed with at least one weld protrusion, and wherein the nut body includes hardened material and the weld protrusion is an annealed portion. According to this aspect of the invention, the entire nut body, including the weld protrusion, is preferably formed from the same hardened material and the weld protrusions are then selectively annealed to form the annealed portion.
A method of correcting misalignment between a stud and a nut of a threaded fastener assembly is also disclosed. In accordance with this aspect of the invention, the method comprises relatively inserting a threaded end of a stud into a radiused or chamfered entrance opening of a pilot member disposed forwardly adjacent the nut. If there is misalignment, the stud threads contacting the radius or chamfered opening will not cross thread therewith. The threaded end of the stud is then relatively axially advanced through the pilot member which is not correspondingly threaded in relation to the stud thread. This relative axial advancement occurs over a predetermined axial distance that forces the stud axis to enter into substantially collinear alignment with the thread axis of the nut before the stud thread affects threaded contact with the nut thread. Relative rotation between the nut and stud will subsequently cause proper threaded engagement to occur without cross threading.
Other and further objects, features and advantages will become apparent from the following description of the presently preferred embodiment of the invention.